Digest of Current Literature. 243 



A Further Inquiry into the Limits of Microscopic Vision, 

 AND THE Delusive Application of Fraunhofer's Optical Law 

 OF Vision. — Dr. Royston-Pigott. — Dr. Pigott does not believe that the 

 formulae of Abbe & Helmholtz have fully determined the limit of 

 visibility of fine lines, and in this paper he gives the results of some 

 investigations, by means of minute miniatures, which show that it is 

 possible to distinguish a bright space between black lines not greater 

 than 1-230,000 or even 1-300,000 of an inch. 



On Some Recent Forms of Camera Lucida.— Frank Crisp. — 

 Several forms of this instrument are described and illustrated without 

 any attempt at criticism or comparison. The forms described are those 

 devised by Hofmann, Pellerin, Swift, and Russel. 



Description OF a New Form of Camera Lucida. — J.C. Russel. — 

 The image formed by the objective of the microscope is reflected 

 through a short tube at a right angle to the axis, and then into the field 

 of the eye-lens of a telescope. The paper and pencil are viewed through 

 this telescope, properly inclined, at the same time. 



Immersion Illuminators. — J. Mayall, Jr. — Mr. Mayall reviews this 

 subject and briefly mentions various appliances which have been used 

 as immersion illuminators. 



Note on A Revolver Immersion Prism for Sub-Stage Illumin- 

 ation. — James Edmunds. — This is a prism of special construction, 

 having four surfaces normal to light incident, at 30°, 41°, 60°, and 49° 

 from the optical axis of the objective. 



A Catoptric Immersion Illuminator. — J. W. Stevenson. — This 

 was devised in 1877, and consists of a plano-convex lens, " worked on a 

 i-inch tool, and having a diameter of 1.2 inches, which is then ' edged ' 

 down to I inch. The upper, or convex side of the lens is cut down or 

 flattened so as to give a surface /^ of an inch in diameter, with which 

 the slide is to be connected, when in use, by a drop of oil or water." 

 The convex surface is silvered. Directly beneath the flattened part an 

 opaque stop of corresponding size is placed. Incident rays, normal to 

 the plane surface, impinge on the curved, silvered surface, and are 

 thrown back upon the plane surface, from which they suffer total re- 

 flection, and converge to a focus, giving a numerical angle of 1.30=120° 

 in balsam. 



The Thallus of the Diatomace^. — F. Kitton. — From a paper by 

 Dr. M. Lanzi, elsewhere noticed. 



JOURNAL DE MICROGRAPHIE. 



December, 1878. 



Under the " Review " we find a reproduction of a note on the Ger- 

 mination OF the Spores of Volvox Dioicus, from F. Henneguy. 



Having previously communicated a note to the Academie des Sciences 

 relative to the sexuality of Volvox dioicus, Cohn, the male appearing 



